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It may be a simple or obvious question, but I'm confused what's changed between those.

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That would be an example of a non structural rivet, does the bottom half of the mandrel steam inside the hollow rivet fall out, unlike structural where it stays, thus strengthening the rivet due to it not being hollow?

Bit confused between both.

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PopRivets (tm) are a simple example of non-structural rivets. When assembled the steel core breaks off leaving a hollow rivet. These cannot be considered to be structural since the forces needed to destroy the joint are acting on a very small cross sectional area and failure is possible in shear, tension and torsional modes. They are appropriate for assembling a surface panel or decorative element to a design. Where significant loadings can occur, of course a denser rivet pattern will be needed, preferably following the distribution of load in the structure.

A structural rivet is usually comprised of a single piece of material that is deformed to bind the parts together. Some hot riveting systems rely on the contraction of the cooling fastener to hold the parts together. Unlike PopRivets there are also cored and coreless versions available in a variety of materials with the necessary properties. Again the rivet pattern is important and some effort should be applied to understanding the optimum distribution.

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Pop Rivets are a fastener composed of two portions: the mandrel and the hat. To install a pop rivet a hole is drilled into the materials, and then the hat is placed into the hole. A rivet installation too is then slid onto the exposed mandrel end. As it begins pulling the mandrel, it deforms the back of the hat creating a strong and quick hold. Once a certain pressure is reached, the mandrel snaps leaving the hat deformed and in place.

Basically, and using the terminology from your diagram, the pulling stem is pulled until exceeding its capacity. During this pulling period the rivet shell deforms and creates a header on the rear of the installation. Once the pulling stem is pulled too tightly it will snap resulting in the stem break point being just below the surface of the rivet shell. This creates a firm secure hold.

Pop rivets are most commonly used for their quick installation process and because you only need access to one side of the installation materials earning them the name blind rivets.

Structural rivets are a bit different as there is a built in locking mechanism that ensure the mandrel portion that stays inside the rivet does not fall out but actually creates a secondary wall. This increases both the rivets strength and hold over time.

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